1. Field of the Invention
The present invention relates to a device for detecting the presence and absence of a person in a certain space, and more particularly to a person detecting device provided with means for sensing capacitance between a pair of electrodes facing to each other with a space, such as a compartment space of a vehicle in which a person is seated.
2. Description of the Prior Art
Recently, it is required to control various equipments in a vehicle depending upon whether or not a person is present in a compartment, or whether or not a part of the person's body is interposed in a space in the vehicle (hereinafter it is merely referred to as the presence and absence of a person), such as a seat-belt device automatically operated when a person gets on the vehicle, and a power window or a locking device automatically operated when a person gets off the vehicle. Accordingly, it is necessary for the vehicle to be provided with means for detecting the presence and absence of a person in the compartment. In Japanese Patent Laid-open Publication Nos. Sho 62-91882 and Sho 62-138780, it has been proposed to employ a conductor provided on a seat and a vehicle body as electrodes forming a capacitor by these electrodes and an inter-electrode compartment space in which the person is seated, and to sense the inter-electrode capacitance varying depending upon the presence and absence of a person in the compartment. In the latter Laid-open Publication disclosed is a device having an oscillator provided externally with the capacitor to detect the presence and absence of the person in the compartment through a change in the oscillation frequency in response to a change in the capacitance of the capacitor, as shown in FIG. 7. When the person takes a seat, the capacitance of a person sensing capacitor C.sub.FR is increased, so that the output frequency of the oscillator is reduced. Then, output data from a counter is compared with a reference value by a digital comparator. Accordingly, when the output data is less than the reference value, a signal "0" representing the presence of the person is provided.
However, in the prior art device as noted above, a so-called self oscillation is used, so that if a noise signal joins a pulse signal fed to the counter through the electrodes or the like, it is not possible to distinguish the noise signal from the pulse signal. Thus, this person detecting device may cause an error in operation. For example, when a noise at a frequency of 10 KHz joins the pulse signal through the electrodes of the person sensing capacitor, the frequency of 10 KHz is counted by the counter. In this case, if the reference value within a frequency range from 5 to 15 KHz is set to represent the presence of the person, the comparator will provide the signal representing the presence of the person. Accordingly, means for excluding such a continuous noise signal is needed, whereas it is extremely difficult for the device employing the oscillator of the self oscillation to be provided with such means.